Natural gas is bought and sold based on its heating value. It is the BTU content that determines the monetary value of a given volume of natural gas. This BTU value is generally expressed in decatherms (one million BTU). In the determination of total heat value of a given volume of gas, a sample of the gas is analyzed and from the composition its heat value per unit volume is calculated. This value is generally expressed in BTU/cu ft. The typical range of transmission quality gas ranges between 1000 and 1100 BTU/cu ft. Production gas, storage facility gas, NGL, and new found Shale Gas can have much higher heating values up to or even exceeding 1500 BTU/cu ft.
There has been a long standing controversy between gas producers and gas transporters regarding entrained liquid typically present in most high BTU/cu ft. gas (rich or “wet” gas). Transporter tariffs require essentially liquid-free gas. Hydrocarbon liquid in the gas being transported causes operational and safety problems. The practice is to separate the liquid before entering a transport (pipe) line.
The API 14.1 standards (Manual of Petroleum Measurement Standards, 2006) scope does not include supercritical fluid (dense phase) or “wet gas” “(a term referenced by the Natural Gas industry as a gas that is at or below its hydrocarbon dew point temperature and/or contains entrained liquid), nor does the GPA 2166 standard (Obtaining Natural Gas Samples for Analysis by Gas Chromatography, 2005). In summary, there is no known standard which defines how to obtain a “representative sample” of a natural gas supply having entrained hydrocarbon in any form.
Therefore to fully comply with the current industry standards, there exists a compelling need to effectively prevent entrained liquids from entering sample systems. Membrane-tipped probes such as the A+ Corporation Genie Probe (see U.S. Pat. No. 6,357,304, U.S. Pat. No. 6,701,794, U.S. Pat. No. 6,904,816, U.S. Pat. No. 7,004,041, and U.S. Pat. No. 7,134,318) have been used for many years to shed entrained liquids inside pressurized pipelines. However, these systems can be overwhelmed with excessive liquid loading, causing the maximum allowable differential pressure to be exceeded, which tends to force liquids through the coalescing elements, and into the sample system.
The differential pressure needed to force liquids through the coalescing elements is a function of the surface tension of the liquid as well as the construction of the coalescing element. This can be further complicated by the use of various liquid chemicals, which are routinely injected into the process, gas such as corrosion inhibitors, amine and carbon dioxide inhibitors, as well as chemicals meant to dry the gas, like alcohols and glycols.
These liquid chemicals may have low surface tensions and may penetrate coalescing elements, in which case said liquid chemicals may combine with the sample, or lower the surface tension of entrained liquids at the coalescing membrane, making it easier for the said undesired liquids to penetrate and get past some coalescing elements. Also, some coalescing elements may have temperature limitations, and thus may be impractical for some applications.
Further, coalescing membranes or the like may have to be changed periodically as a maintenance precaution to insure reliable operation. Accordingly there is a need for a physical pre-filter to eliminate the bulk of the liquid entrained in the gas which would operate in a variety of conditions with little maintenance, and which is more reliable in operation than current systems.
Cyclonic separation techniques have been utilized in a variety of capacities for over 100 years. A typical cyclonic separator channels a fluid stream through a housing having a geometry formed to generate a vortex, exploiting centrifugal force with gravity and pressure differentials to separate liquid particles from gaseous streams, as well as other applications.
Cyclone-type pre-filters have been used for many decades. For example, D. W. Birnstingl describes a measuring head for a conductivity meter combining a conductivity cell with a cyclone filter to filter liquid in U.S. Pat. No. 3,471,775 filed in 1966.
UOP Inc. of Des Plains, Ill., describe a sampling probe that uses a V-shaped shield to pre-filter particles from sample (see U.S. Pat. No. 4,481,833 from 1984). Another company, Anarad Inc. of Santa Barbara, Calif., describes a filter probe for stack gas that uses an inertial filter with a constant bypass flow requirement to remove dust without clogging (U.S. Pat. No. 5,237,881 from 1993). The University of Akron describes a cyclone collection vessel combined with filter media for separation of a suspension (U.S. Pat. No. 6,210,575 from 2001). M & C Products Analysis Technology, Inc. of Ventura, Calif., describes an in situ particle separation system with filter media for separating particles from gas samples (U.S. Pat. No. 7,337,683 from 2008).
More recently, the General Electric Company of Schenectady, N.Y. describes sample probe for removing particles from a gas stream using a shield and a flow reversal technique (U.S. Pat. No. 8,087,308 from 2012). These devices are not used to remove entrained liquids from gas samples. No one in industry has contemplated using the cyclone technology to solve the problem of entrained liquids in natural gas samples.
Dekati Ltd of Finland offers the CYCLONE brand cyclonic separator for removal of large particles from a Sample Stream. This device is designed to be placed in a flue gas flow in a stack as well as exterior to the stack. In either instance, an isokinetic sampling probe is utilized to draw the sample. Various isokinetic nozzles are available and may be utilized interchangeably, depending upon the circumstances of use.
Other types of fluid separators may include:                Filter Vane Separators utilize a structure comprising a series of plates or baffles along a passageway to exploit inertial impaction of the fluids, combined with gravity, to facilitate separation.        Centrifugal separators which utilize centrifugal forces to separate the heavier fluid droplets or particles from the gas stream. Cyclone separators operate on this principal as well as well as knock-out drums.        Liquid/Gas coalescer cartridges, filters, membranes and the like are generally not suitable for removal of liquids in bulk, relying upon inertial impaction, the particles engaging a fibrous mass in a container which may include an indirect pathway utilizing inertial impaction and gravity to collect and drain fluid.        Mist Eliminators likewise rely upon the principal of inertial impaction, but instead of plates or baffles, relying upon fibers, meshes or the like.        
A+ Corporation makes a self-cleaning filter under the trademark TORNADO (for example, model 602). It is an external cyclone type filter. Another external cyclonic filter is made by Collins Products Co, maker of the SWIRLKLEAN brand bypass filter which also uses a cyclone-type filter external to the pipeline, situated upstream the analyzer, although the SWIRLKLEAN system does not utilize gravity separation, instead exploiting a bypass technique as detailed at http://www.collins-products.com/.
To summarize, the prior art teaches various systems for removing liquid particulates or the like from a gaseous fluid stream. Removal of such entrained liquid is imperative as a component of gas analysis as detailed above, although such systems are imperfect and many designs can be overwhelmed by a liquid slug or the like. Anytime liquid is removed from the source and transported into the sample system, the liquid distorts the true composition of the sample.
However, due to shortcomings in the above systems there remains a long felt, but unresolved need for a system with the ability to reliably prevent liquids from overwhelming the separator to prevent liquid from entering the sample system, thus preventing sample distortion and contamination, which can equate to wrong analysis and very costly incorrect monetary exchanges. This is especially true at custody transfer points for both producers and transporters.
It would therefore be an improvement over the art to provide a system which does not require a bypass stream and with the required venting to the atmosphere or the ground, an undesirable requirement of several of the above referenced systems.